The present invention generally relates to optical storage units, and particularly relates to a storage unit which can automatically and accurately adjust an optical phase compensation quantity at a high speed in a relatively simple manner, so that information can satisfactorily be reproduced from the recording medium while crosstalk from an adjacent track of a recording medium is reduced.
Presently, optical recording mediums are popularly used as recording mediums capable of recording and reproducing audio signals and image signals thereon and therefrom. Particularly, research and development efforts are currently being made with respect to magneto-optical recording mediums and phase change recording mediums which are promising rewritable high-density recording mediums.
In order to improve the recording density of the optical recording medium which records information on a spiral track or concentric tracks, there are two conceivable techniques, namely, reducing the track pitch and improving the linear recording density. Both of these techniques can be realized by reducing the wavelength of a laser beam emitted from a semiconductor laser which is used for the recording and reproduction. However, an inexpensive semiconductor laser which can emit a stable green or a blue laser beam at room temperature for a prolonged period of time has yet to be developed. Under these circumstances, there are proposals to greatly improve the recording density using a conventional laser beam such as a magnetic super resolution (MSR) laser proposed for the magneto-optical recording medium.
In the case of a rewritable memory medium such as the magneto-optical recording medium, the light beam used for information recording and the light beam used for reproduction have the same wavelength.
On the other hand, in the case of a read only memory (ROM) medium information in the form of phase pits is prerecorded on the recording medium using a special gas laser or the like having a shorter wavelength than that used to record information in a rewritable memory medium.
The ROM medium possesses a higher recording ratio than a comparable rewritable medium because information is recorded using a shorter wavelength. However, the high cost of the necessary laser makes it uneconomical to utilize this type of gas laser in a rewritable drive.
Another approach to increasing the storage capacity of a disk medium is to increase the number of tracks by, for example, providing tracks in both land and groove portions of the medium (land-groove type recording medium). Typically, tracks are provided on either a land or a groove portion of a recording medium. In contrast, tracks are provided in both land and groove portions in the land/groove type recording medium. Consequently, the recording density of the land-groove type recording medium is double that used in typical recording media, provided that the same linear recording density and the track pitch used. For this reason, the land-groove recording is an extremely important technique from the point of view of developing a high-density recording medium.
In the magneto-optical recording medium art, there are reports that the above-described MSR approach is not only capable of improving the linear recording density but is also capable of reducing crosstalk between the tracks. Accordingly, studies are being made to apply th MSR to land and groove recording. However, the conditions for realizing the MSR are extremely complicated, the stability is still an unknown factor, and the cost is high. For example, the conditions for realizing the MSR include the dependency of the reproducing laser power to the linear velocity, the need for a reproducing magnetic field in some cases, and the use of a recording medium having at least three magnetic layers in some cases.
In a land type recording medium, information is only recorded on land portions, and a groove (on which no information is recorded) separates adjacent lands. Importantly, the lands reduce the generation of crosstalk in which the information recorded on two adjacent lands mix during the reproduction of information.
Similarly, in a groove type recording medium, information is only recorded on the groove portions, and a land (on which no information is recorded) separates two adjacent grooves. In this type of medium, the lands suppress the generation of crosstalk in which the information recorded on two adjacent grooves mix during the reproduction of information.
In contrast, in a land-groove type recording medium, information is recorded on both land and groove portions. Consequently, the information recording regions are adjacent to each other, and the generation of crosstalk from the adjacent recording region is unavoidable. As a result, the crosstalk from the adjacent recording region greatly effects the information reproducing characteristic.
For example, a Japanese Laid-Open Patent Application No. 8-7357 proposes a method of reducing the crosstalk by appropriately selecting the depth of the groove. According to this proposed method, the wavelength of the light beam is 680 nm, the numeric aperture (NA) of the objective lens used is 0.55, and the width of the land and the groove are 0.7 xcexcm, for example. Notably, the crosstalk can be reduced by selecting the depth of the groove to be approximately ⅙ of the wavelength.
However, the above-described method is unable to sufficiently reduce crosstalk, if the track pitch is reduced while maintaining constant the size of the beam spot of the light beam. Moreover, as described above, reducing the size of the beam spot would require an uneconomically more expensive laser. In addition, when the depth of the groove is approximately ⅙ the wavelength, the carrier level of the signal decreases to an undesirable level, and the level of a push-pull signal which is used as a tracking error signal similarly decreases. Further, it has been reported that the above-described method""s ability to reduce crosstalk is significantly affected by changes in Kerr ellipticity, focal point error of the objective lens, spherical aberration and the like.
Japanese Laid-Open Patent Application No. 9-128825 proposes a method of reducing the crosstalk by setting the width of the land to be approximately equal to the width of the groove which has an optical depth of approximately xe2x85x9 the wavelength. However, this latter proposed method requires that two read channels be provided. Namely, one read channel for reproducing the signal from the land and another read channel for reproducing the signal from the groove. Moreover, a plurality of wave plates and beam splitting prisms or the like must also be provided. As a result, a device incorporating this method becomes complex and the optical information storage unit becomes prohibitively expensive.
Furthermore, when recording information on a land or a groove of the magneto-optical recording medium, the reflected light component from an adjacent groove or land increases as the track pitch becomes smaller, thereby introducing a phase error and a change in the polarization state. Consequently, there was a problem in that the information cannot be reproduced from the magneto-optical recording medium in a satisfactory manner.
On the other hand, a Japanese Laid-Open Patent Application No. 9-161347 proposes carrying out a different phase compensation with respect to the polarization component with respect to the signal reproduced from the land and the signal reproduced from the groove. However, this reference fails to disclose how to accomplish switching of the phase compensation quantity with respect to the polarization component with respect to the signals reproduced from land portions versus signals reproduced from groove portions. For this reason, there exists a demand to realize a technique for automatically adjusting the phase compensation quantity in a simple manner, at a high speed, and with a high accuracy.
Therefore, it is an object of the present invention to provide a storage unit which can automatically and accurately adjust an optical phase compensation quantity at a high speed in a relatively simple manner, so that a crosstalk from an adjacent track of a recording medium is reduced and information can satisfactorily be reproduced from the recording medium.
The above-described object can be realized by a storage unit which uses a light beam to record information on and/or reproduce information from a recording medium having tracks formed in both land and groove regions thereof. The storage unit includes a reproducing mechanism (optical pickup) for reproducing information from the recording medium, and am optical phase adjuster for adjusting a phase of a polarization component of the light beam reflected from the recording medium in accordance with the land/groove type of the track being accessed. According to the present invention, it is possible to automatically and accurately adjust the optical phase adjusting quantity at a high speed in a relatively simple manner.
According to one aspect of the invention, distinct optical phase adjusting quantities are used to adjust the phase of the polarization component of the light beam for accessing information from a land and a groove, and the distinct optical phase adjusting quantities are stored in a control region of the disk medium. Moreover, the control region stores information identifying a track type (land or groove) for each address. The optical phase adjusting quantities and the track type information are accessed when the disk medium is initially loaded in the storage unit. In operation, the storage unit determines the appropriate phase adjusting quantity in accordance with the track type information of a target address.
Alternatively, the above-described optical phase adjusting quantities and/or the track type information may be stored in a memory in the optical storage unit. In this event, the storage unit determines the disk type when the disk medium is initially loaded, and selects the appropriate phase adjusting quantity in relation to whether the accessed address is a land or a grove.
The optical phase adjuster may include a wave plate, and a mechanism which adjusts an inclination angle of the wave plate with respect to the light beam obtained via the recording medium, based on the optical phase adjusting quantity.
Alternatively, the optical phase adjuster may include an electro-optic element, and a circuit which controls a signal that drives the electro-optic element based on the optical phase adjusting quantity. According to the present invention, it is possible to accurately adjust the optical phase adjusting quantity by use of an optical system having a relatively simple construction.
Therefore, according to the present invention, it is possible to automatically and accurately adjust the optical phase adjusting quantity at a high speed in a relatively simple manner, and the crosstalk from the adjacent track of the recording medium can be reduced.